Spark ignition of an air/fuel mixture within a combustion chamber of an internal combustion engine generally involves igniting the air/fuel mixture with an electric spark jumped between an electrode and a ground electrode of a spark plug. An alternative to spark ignition known in the art is torch jet-assisted spark ignition which, as taught by U.S. Pat. Nos. 3,921,605 to Wyczalek and 4,924,829 to Cheng et al., offers several advantages over spark ignition approaches. As the name suggests, torch jet-assisted spark ignition utilizes a jet of burning gases which is propelled into the combustion chamber in order to increase the burning rate within the combustion chamber by providing increased turbulence as well as presenting a larger flame front area. As a result of a faster burning rate, lower cyclic variation in cylinder pressure is achieved, which enables a higher engine efficiency with a higher compression ratio.
In a torch jet-assisted spark ignition system, the jet typically emanates from a combustion prechamber, and passes through an orifice into the main combustion chamber. Though an air/fuel mixture can be introduced directly into the prechamber through a separate intake valve or fuel injector, it is generally preferable that the air/fuel mixture originate from the main chamber in order to simplify the construction of the engine and its ignition system. Furthermore, combustion of the air/fuel mixture within the prechamber can be initiated from within by a separate igniter, or can be initiated by the flame front within the main chamber. With either approach, combustion typically proceeds relatively simultaneously in both the prechamber and the main chamber. However, because of the small relative volume of the prechamber, a high pressure is developed in the prechamber while the pressure is still relatively low in the main chamber. As a result, a jet of burning gases shoots from the prechamber far into the main chamber, and thereby significantly increases the combustion rate in the main chamber.
A torch jet spark plug taught by copending U.S. patent application (Attorney Docket No. G-11389) to Durling et al., assigned to the assignee of this invention, offers particularly advantageous features which enhance performance as well as manufacturability. One improvement is a greater resistance to pre-ignition within the prechamber at high operating temperatures. In particular, the spark plug employs a pair of inner electrodes which form a radial spark gap within the prechamber. The inner electrodes are positioned within the prechamber so as to be spaced away from the engine's combustion chamber. Furthermore, the spark plug's construction provides for intimate thermal contact between the inner electrodes and the body of the spark plug, so as to promote thermal conduction therebetween, which further minimizes the inner electrodes' operating temperature, and thereby reduces the likelihood of preignition. The spark plug taught by Durling et al. is also advantageous in that it substantially eliminates the potential for internal short circuits to ground within the spark plug as a result of deposits building up on the internal surface of the prechamber.
Durling et al. achieve the above advantages by forming within the insulator body of the spark plug a combustion prechamber on whose surface an inner electrode is formed. The prechamber has a first end and an oppositely disposed second end which are preferably disposed along the longitudinal axis of the insulator body. An orifice is formed in the insulator body at the second end of the prechamber such that, when the spark plug is properly installed in an engine, the prechamber is vented to the engine's main combustion chamber through the orifice. A center electrode is mounted in the insulator body so as to project into the first end of the prechamber, while an outer electrode is formed integrally with the orifice at the second end of the prechamber. A ground electrode is disposed adjacent the outer electrode so as to define an outer spark gap therewith.
A metal ink, preferably a catalytically-active metal such as platinum suspended in a suitable carrier, is deposited on the internal surface of the prechamber to form the inner electrode. The inner electrode circumscribes the center electrode so as to form an inner radial spark gap therewith. The inner electrode is also in electrical contact with the outer electrode, so as to be able to deliver an electric current from the center electrode to the outer electrode. In one embodiment, the metal ink is deposited as a longitudinal stripe along the inner surface of the prechamber, while in another embodiment, the metal ink is deposited on substantially the entire longitudinal surfaces of the prechamber as well as the rim of the orifice, so as to simultaneously form a hollow inner electrode and a hollow outer electrode. With both embodiments, the inner electrode forms an electrical capacitor with the spark plug's metal shell and the insulator body positioned between the inner electrode and the metal shell. The greater electrode surface area provided by the inner electrode of the second embodiment serves to enhance the capacitive effect. As a result of capacitive charging, the electric sparks which occur at the inner radial spark gap and the outer spark gap will fire sequentially rather than simultaneously, so as to reduce the peak voltage levels required to fire the spark plug. Accordingly, the electrical demands placed on the ignition coil and wiring will also be reduced.
While the structure taught by Durling et al. is advantageous for the reasons noted above, depositing the metal ink on the interior surface of a relatively small prechamber is difficult. In addition, depositing the metal ink after the insulator body and its prechamber have been formed and fired necessitates that two heating steps be performed--a first to fire the "green" ceramic blank from which the insulator body is formed, and a second to dissipate the carrier component of the metal ink and sinter the metal ink. Finally, with the above technique, the metal ink does not significantly penetrate the ceramic blank, such that the resulting inner electrode is susceptible to erosion from the hot gases within the prechamber. Confronted with a somewhat different problem, U.S. Pat. No. 5,210,458 to McDougal discloses a method by which an electrically conductive path can be formed through a solid dielectric material, such as a center electrode in a spark plug insulator body, using a cermet ink. However, McDougal teaches a technique in which the cermet ink is introduced into a mass of ceramic powder which is under a continuous compressive force, such that the ceramic powder inherently collapses about the cermet ink, thereby forming a solid electrode running through the body formed from the ceramic powder. In contrast, the inner electrode utilized in the spark plug taught by Durling et al. is disposed on the internal surface of a prechamber within the insulator body of the spark plug. Accordingly, the inner electrode taught by Durling et al. cannot be formed in accordance with the teachings of McDougal.
The method taught by McDougal involves applying the cermet ink to a spindle which is then inserted into a rubber mold containing granulated ceramic material. To prevent removal of the cermet ink during insertion, McDougal uses a flash drying operation to dry the cermet ink on the spindle. While the above approach may be suitable when the object is to form a discrete mass of conductor material within a ceramic body, as is the goal of McDougal, it has been determined that a dried ink will not transfer well to the ceramic material using certain molding techniques. More specifically, when forming an electrode on a surface, as taught by Durling et al., it is highly desirable that the ink impregnate the ceramic material so as to become substantially integral with the ceramic material, such that a strong bond between the electrode and the ceramic material is created. The method taught by McDougal cannot reliably achieve this desirable result. In addition, the method taught by McDougal requires pressure to be maintained as the spindle is removed from the mold in order to effectively remove the cermet ink from the spindle, a requirement which somewhat complicates the molding process. Furthermore, the use of an additional operation to flash dry the ink on the spindle is disadvantageous from a manufacturing standpoint.
Therefore, what is needed is a method for forming an inner electrode on the internal surface of a chamber within the insulator body of a torch jet spark plug, wherein the method is relatively uncomplicated and necessitates a minimal number of operations, so as to be suitable for use in the mass production of torch jet spark plugs for an internal combustion engine.